The magnetic flux density (B) is the magnetic flux per unit area of a magnetic field at right angles to the magnetic force. Magnetic flux density (B), also known as the magnetic induction, is a vector quantity. It is one of two quantities that specifies both the strength and direction of the magnetic field; the other is magnetic field strength (H). B and H are related as follows:

 

    B = μH

 

where μ is the permeability.

 

This corresponds to the electrical analog:

 

    D = εE

 

where D is the electric flux density (also known as the electric displacement, E is the electric field strength (electric intensity), and ε is the permittivity.

 

This correspondence, however, a legacy from magnetostatics, is misleading. In a dielectric medium the electrical force exerted upon a free charge is proportional to E, not D. On the other hand, in a magnetic material, the magnetic force that is exerted upon a moving charge is proportional to B, not H. Thus, in the sense that E is the electric field for free charges, B is the magnetic "field" for moving charges. Unfortunately, H bears the title that describes B's function.

 

The magnetic flux density can also be defined in terms of the effects the field has, for example, by B = F / qvsinθ, where F is the force a moving charge q would experience if it was traveling at a velocity v in a direction making an angle θ with that of the field.

 

The SI unit of magnetic flux density is the tesla, which is equal to one weber per square meter (W m^–2).